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Unit 9: Electricity

Measuring the flow of electricity — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 12 2 quizzes

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Lesson summary

Measuring the flow of electricity — HIA INTERNATIONAL notes: State the unit of current, connect an ammeter correctly and apply the rule for current in a series circuit.Current is measured in amperes (amps) — the symbol is A.Small currents are measured in milliamps (mA) — 1 A = 10…

  • When electrons move faster the current is greater
  • An ammeter measures current
  • An ammeter has two terminals
  • The red terminal must be connected
  • In a circuit diagram
  • An ammeter is always connected in series

Section 1 of 9Units of current

Units of current
Current is measured in amperes (amps)
  • Current is measured in amperes (amps) the symbol is A.
Small currents are measured in milliamps (mA)
  • 1 A = 1000 mA.
When electrons move faster the current is greater
  • When electrons move faster the current is greater when they move more slowly the current is smaller.

Section 2 of 9The ammeter

1An ammeter measures current
  • An ammeter measures current there are different types, but all do the same job.
2An ammeter has two terminals
  • An ammeter has two terminals one red and one black.
3The red terminal must be connected
  • The red terminal must be connected directly, or through other components, to the positive terminal of the power supply.
4In a circuit diagram
  • In a circuit diagram both ends of the ammeter symbol are the same.

Section 3 of 9Connecting an ammeter

1An ammeter is always connected in series
  • An ammeter is always connected in series with the other components.
2In series means end to end
  • In series means end to end one after another, with no branches in the circuit.
3The current flows through the ammeter
  • The current flows through the ammeter so the same current flows through it as through the component.
4Do not connect an ammeter directly to a cell
  • Do not connect an ammeter directly to a cell there must be a lamp or other component in the circuit.

Section 4 of 9Reading an ammeter

Reading an ammeter
Read the scale at the pointer
  • Read the scale at the pointer and include the unit in the answer.
Work out the size of each division
  • Work out the size of each division before reading values between whole numbers.
Check the scale range
  • Check the scale range the scales on different ammeters differ.

Section 5 of 9Current in a series circuit

Current in a series circuit
The current is the same at every point
  • The current is the same at every point an ammeter placed at any position gives the same reading.
Current is not used up
  • Current is not used up so it does not decrease as it passes the components.
Components can be put in a different order
  • Components can be put in a different order and the current will be the same.

Section 6 of 9Measuring voltage

Measuring voltage
Voltage
Voltage a measure of the energy given to, or transferred by, the charge.
Voltage is measured in volts (V)
  • Voltage is measured in volts (V) with a voltmeter connected in parallel across the component.
Resistance
Resistance how much a component opposes the current, in ohms (Ω); R = V ÷ I.

Section 7 of 9Series and parallel current rules

Series circuit
  • Series circuit one loop, the same current everywhere, and adding lamps makes each lamp dimmer.
Parallel circuit
  • Parallel circuit the current splits at a junction, and the branch currents add up to the current from the cell.
In a parallel circuit each lamp can be switched separately and stays bright.

Section 8 of 9Key terms

TermMeaning
Ampere (amp)The unit of electric current, symbol A.
MilliampOne thousandth of an amp, symbol mA.
AmmeterA meter connected in series to measure current.
In seriesConnected end to end with no branches.
VoltmeterA meter connected across a component to measure voltage.
ResistanceHow much a component opposes the flow of current, in ohms.
VoltThe unit of voltage, symbol V.
JunctionA point in a circuit where the current splits or joins.

Section 9 of 9Exam technique

1
Connecting the ammeter in parallel — an ammeter must be in series.
2
Forgetting the unit when giving an ammeter reading — write the number with A or mA.
3
Saying current is used up by the first lamp — current is the same all round a series circuit.
4
Mixing up the red and black ammeter terminals — the red terminal goes toward the positive terminal of the supply.

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